Pressure sensitive adhesives based upon polyacrylates or "acrylics" have been, and continue to be, the choice for marking films used in the graphics arts industry. Films having acrylate adhesives are well-known for their unique balance of tack, peel strength, shear holding power and excellent resistance to weathering agents such as sunlight, heat, actinic light, road grime, moisture, oxidation and cleaning agents. Films having acrylic adhesives are very useful where high-performance, durable graphics are desired. Many commercial examples of marking films having acrylic adhesives are known in the art. These include those sold by Minnesota Mining and Manufacturing Company, St. Paul, Minn. (hereafter "3M") under the Scotchcal.TM. and Controltac.TM. brand names.
The use of self-adhesive graphics, made from pressure sensitive adhesive films, has many advantages over direct painting or image spraying. Among these advantages are the wide range of decoration conditions possible, remote site decoration, and better image quality and consistency. However, pressure sensitive adhesive graphics can exhibit high tack when working temperatures exceed about 80.degree. F. (27.degree. C.) making installation of large sheets difficult.
A number of patents have addressed the issue of too much tack at high installation temperatures. U.S. Pat. No. 3,314,838 (Erwin) teaches that partially embedding hollow, non-adhesive spheres into the PSA surface renders the adhesive film non-tacky upon contact. Subsequent application of pressure to the film, for example, by pressing with a squeegee, breaks and/or compresses the spheres into the adhesive, instantly allowing the adhesive to contact the substrate and form a bond.
U.S. Pat. Nos. 5,141,790 (Calhoun et al.) and 5,296,277 (Wilson et al.), the teaching of both of which are incorporated herein by reference, further teach that other types of projecting particles, clusters of particles, and adhesive surface features can similarly function to overcome problems of excessive tackiness and preadhesion. These adhesive surface features are particularly useful for extending the useful application temperature range of a PSA film to above 100.degree. F. (38.degree. C.). Commercial examples of such self-adhesive marking films having extended upper working application temperature ranges have been sold by 3M under the Scotchcal.TM. and Controltac.TM. brands for many years.
The above examples show that means exist to extend the useful installation temperature range of a PSA film. The ability to apply markings in cold temperature conditions would further expand the usefulness of PSA marking films for remote site application. Adhesive films from a variety of manufacturers are known to have sufficient tack to enable applications to be made at 50-60.degree. F. (10-16.degree. C.) and for limited numbers of products as low as about 40.degree. F. (4.4.degree. C.). For example, 3M's Series 180, Series 160 and Series 3500 films may all be installed at temperatures as low as 40.degree. F. (4.4.degree. C.) to certain surfaces.
However, there still exists a need for new self-adhesive films that are broadly useful over a wide range of surfaces and have sufficient low temperature tack to allow reliable applications to be made at temperatures of 20-30.degree. F. (-7 to -1.degree. C.) or even colder. Such adhesive films would have the potential to save significant installation costs by reducing site and substrate preparation time, lost service time, site overhead, and reinstallation.
For example, self-adhesive markings are commonly used to decorate over-the-road tractor trailers at a wide variety of sites. The trailer surface (substrate) is washed and cleaned to remove grease and oils and then dried. If the trailer is cold, it is heated in the garage environment until it reaches a temperature that will accept application of the graphic markings. The trailer must be maintained at temperatures about 40-60.degree. F. (4.4-16.degree. C.) during the marking application process to enable the adhesive to instantly bond or back to the trailer surface with sufficient strength to keep the marking in place when the trailer is placed back into service. However, in cooler environments, particularly during cooler months of the year, heated garages are required for reliable marking installation, and such access can be expensive. Installation at local service shops and trailer yards would be preferred and less costly. It is therefore highly desirable to have markings with PSAs that are sufficiently tacky to be applied to the temperatures at temperatures at least as low as 20-30.degree. F. (-1 to -7.degree. C.) or lower. Such films would minimize the need for and costs associated with heated application facilities.
European Patent Application 0 593 231, (Carleton), discloses a method for improving the performance of pressure sensitive adhesives below about -17.7.degree. C. (0.degree. F). The method relates to the addition of an ethylene oxide/propylene oxide block copolymer to the adhesive and produces improved peel strength performance of PSAs in freezer-grade applications. This composition addresses the problem of adhesion loss to packages that are cooled after the PSA label is applied. It does not address the issue of increased cold temperature tack to improve application in a cold environment.
U.S. Pat. No. 5,049,608, (Medina), discloses a PSA composition having good low temperature performance. The PSA is a 2-ethylhexyl acrylate/polar monomer copolymer emulsion and contains a nonionic C.sub.7 to C.sub.18 alkylphenoxypoly(ethyleneoxy) ethanol having at least 70 ethyleneoxy units.
Despite the foregoing, a need still exists for PSA marking films that can be readily applied at low temperatures.